The present invention relates to passive infra-red presence detectors which operate to detect the presence of a target emitting infra-red radiation.
In particular, but not exclusively, the present invention relates to passive infra-red presence detectors for use within traffic control systems.
Traffic control systems operate to regulate the flow of traffic at predetermined junctions within a road system of a town. Typically, traffic control systems take the form of traffic lights operatively connected to a controller. The controller operates to generate instructions which are visually represented by the traffic lights, which thereby provides a means for instructing the vehicles to `stop` or `go` in accordance with a predetermined routine. In order to operate effectively, such traffic control systems must be provided with a means for detecting the presence and movement of vehicles.
Known systems for detecting the presence and movement of vehicles on roads include the use of inductance loops buried in roads over which the vehicles pass. An inductance loop operates to detect the presence of a vehicle from a change in the inductance of the loop, as a result of the presence of a vehicle. Disadvantages with this known system arise from the requirement that the inductance loop must be buried in a road at a position over which the vehicles pass. Inductance loops are expensive to bury and are often disturbed when the road in which they are buried has to be repaired or, alternatively the repair of the road is rendered more difficult. Furthermore, inductance loops are unsuitable for use with metalised roads.
The aforementioned disadvantages with inductance loops have led to a preference for above ground detectors. Such above ground detectors include microwave sensors which operate to detect microwaves transmitted and reflected by vehicles on an approach road to a traffic control system, thereby providing a means for detecting the movement of a vehicle from the reflected microwaves, in accordance with a change in a time between transmission and reception. Such microwave sensors are however expensive. For this reason passive infra-red detectors, which serve to detect the presence of a target vehicle from infra-red radiation emitted thereby, are preferred.
Known passive infra-red detectors use either pyro-electric sensors or thermopile sensors, or a combination of both. A thermopile sensor operates to generate a signal representative of an absolute level of infra-red radiation received from a target vehicle. Thermopile sensors are therefore appropriate for use in detecting static target vehicles, although they may also be used for dynamic targets. However, thermopile sensors are expensive to implement and provide a poor signal to noise ratio. By contrast, pyro-electric sensors are inexpensive and provide a comparatively high signal to noise ratio. However pyro-electric sensors suffer a disadvantage in that they only detect changes in infra-red radiation. Therefore, such pyro-electric sensors are only suitable for detecting movement of a target emitting infra-red radiation and hitherto have been inappropriate for use in detecting stationary targets. For this reason, passive infra-red presence detection systems, which are required to detect both stationary as well as moving targets embody both pyro-electric sensors as well as thermopile sensors.